1. Field of the Invention
The invention relates to switching of both telephony data and packet data within a telecommunication switching system.
2. Discussion of the Prior Art
Time division multiplexed (TDM) switching and packet switching are alternative techniques for multiplexing many low-speed data channels into a single high-speed data channel for transport through a network. TDM data is passed to a telephone network, such as the public switch telephone network (PSTN), and packet data is passed to a packet network, such as the Internet. By multiplexing multiple low-speed data channels into a single high-speed data channel, benefits such as economic advantage and ease of management are realized. Traditionally, TDM networks and switching systems are segregated from packet networks and switching systems because each technique has different data types.
In TDM switching, a fixed partition of the available transport bandwidth is reserved when a connection to the switch is established. The partition takes the form of a “time slot,” which is a segment of the data stream that occurs at regular intervals. During its assigned time slot, a data source may insert TDM data towards the remote end. If the source has no TDM data to send, the time slot is unused. Similarly, there is no mechanism for the data source to temporarily exceed the capacity provided by its assigned time slot. For TDM switching, data transfer characteristics for a given data channel, such as end-to-end delay and delay variation, are bounded and are independent of other channels carried over the same transport facility. Since the available data rate is constant, TDM is well-suited to data types such as uncompressed voice and video that present a constant bit stream to the network and require low, predictable transmission delays.
FIG. 1 illustrates a plan view of a traditional time-space architecture for switching TDM traffic. The terms “time” and “space” refer to the dimensions which are manipulated in order to route each segment of TDM data to its desired destination. The “time” dimension corresponds to the time slots defined for each incoming and outgoing port. The “space” dimension corresponds to the incoming and outgoing physical ports of the switch. Traditional time-space architectures for switching TDM traffic is discussed in: Tarek N. Saadawi, Mostafa H. Ammar, and Ahmed El Hakeem, “Fundamentals of Telecommunication Networks,” John Wiley & Sons, 1994; and Matthew F. Slana, “Time-Division Networks,” chapter in “Fundamentals of Digital Switching” (John C. McDonald, ed.), Plenum Press, 1983.
In FIG. 1, incoming data at each input port is organized into frames 5, which are repetitive, fixed length sequences of time slots 6. During each frame, the time-space switch 7 passes data corresponding to each time slot 6 at an input port 8 to a specific time slot of an output port 9. The correspondence of input port 8 to output port 9 is specified by a stored switch configuration 10, which is generated at the time a connection (e.g., telephone call) is established and is static for the duration of the connection. Each TDM data unit arriving at the input of the switch 7 may have to be stored for a short time period while awaiting output from the switch at the proper time slot. The time period corresponds to the difference in time between the input time slot and the output time slot. The storage interval is constant and is less than one frame in duration.
In contrast to TDM switching, packet switching uses burst-type data, such as those associated with e-mail and file transfer applications. In a packet network, packet data sources compose data into units called packets for passage through the network. When a packet data source has a packet for transmission, it competes with other packet data sources for access to the transport facility. Since a packet data source without data to send consumes no network resources, packet switching is ultimately more efficient in terms of total data throughput. However, packet data sources requiring service (i.e., transmission of data) may be momentarily “blocked” by other packet data sources simultaneously requesting service. Therefore, the time required for a packet to traverse the network is not “deterministic” from the point of view of each packet data source. Furthermore, during periods of overuse, the network may “drop” data packets, and these data packets never reach their destination.
FIG. 2 illustrates a plan view of a traditional architecture for switching packet traffic.
In contrast to the architecture shown in FIG. 1, packets 12 are not organized into frames, and the temporal position of each packet 12 with respect to other packets 12 is unimportant. The destination information for each packet is carried, either explicitly or implicitly, within the packet. Based on this information, the switch 13 routes the packet 12 from the input port 14 to one the output ports 15 of the switch 13. The information for routing the packets 12 is represented by the dashed lines in the switch 13. Unlike the switch 7, there is no guarantee that a collision will not occur at a given output 15 of the switch 13. Therefore, queues 16 are implemented at each output that can temporarily store traffic until the output becomes available. The queues 16 compensate for the variation in instantaneous demand for access to each output port 15. Traditional architectures for switching packet traffic is discussed in: Tarek N. Saadawi, Mostafa H. Ammar, and Ahmed El Hakeem, “Fundamentals of Telecommunication Networks,” John Wiley & Sons, 1994; and Myron J. Ross, “Circuit versus Packet Switching,” chapter in “Fundamentals of Digital Switching” (John C. McDonald, ed.), Plenum Press, 1983.
For a TDM switch, data corresponding to each incoming channel arrives at fixed, predictable intervals, and the input data rate always exactly matches the output data rate. For a packet switch, no specific time intervals are defined, so packets associated with each channel may arrive at any given interface at any time, and the input data rate in a packet switch may exceed the output data rate. These differing characteristics of TDM data and packet data imply the use of disparate architectures for switching, as has been done traditionally. However, it is possible to design a switching platform that is capable of handling a mix of TDM and packet-switched data. Such a platform is termed a “hybrid switch.”
Hybrid switching techniques have been proposed and implemented in which TDM data is encapsulated into packets at the input of a hybrid switch. The TDM encapsulated packet is then routed to its destination output port using traditional packet-switching techniques. At the destination, the TDM data is extracted from the packet and is returned to TDM format. Traditional architectures for hybrid switching are discussed in: Myron J. Ross, “Circuit versus Packet Switching,” chapter in “Fundamentals of Digital Switching” (John C. McDonald, ed.), Plenum Press, 1983.
FIG. 3 illustrates a plan view of an architecture for a prior art hybrid switch. The switch 13 of FIG. 3 operates in the same manner as the switch 13 in FIG. 2. The difference between FIGS. 2 and 3 is the type of packets that are sent to the switch 13. In FIG. 2, the packets 12 only contained packet data, and in FIG. 3, the packets 17 can contain either packet data, designated as “P”, or TDM data, designation as “T”. Prior to being sent to the switch 13, the TDM data is encapsulated as a packet, and after exiting the switch 13, the TDM data is de-encapsulated from the packet.
Prior art hybrid switches are advantageous in that a common packet switch core can be used to route both TDM data and packet data. However, the advantages of low, predictable delays offered by TDM switching may be lost because packets containing TDM data must compete with other packets in passing through the switch. Furthermore, the difficulty of converting the TDM data to and from packet data typically sacrifices the gains won by utilizing a common switching mechanism.